The present invention relates to electron beam accelerators and has particular utility in the high speed irradiation of thin layers of material. Important advantages of the present invention relate to improved control over the dose distribution in the irradiated material and to the ability to provide a high dose at a low dose rate during a single pass of the material.
It is common to irradiate material with accelerated electrons by passing the material, via a conveyor, through an electron beam. Stationary columnar beams, scanning columnar beams, and elongated sheet-like beams have been employed to effect irradiation for various purposes, such as sterilization, paint hardening, and material curing in general. All prior art approaches to electron beam irradiation processes have had a common deficiency, namely the lack of a simple and accurate means by which the dose distribution pattern in the irradiated material could be varied as required by the particular curing process to be performed. For example, a great many irradiation processes, such as paint hardening, require that the irradiation dose be uniformly distributed across the treated material. Others, such as vulcanization of rubber wherein the edges of the material are only partially cured, require specific non-uniform dose distributions. To change from uniform to non-uniform dose distribution, or from one non-uniform dose distribution pattern to another, is impossible or exceedingly difficult in prior art electron acceleration tubes.
For example, consider the stationary columnar electron beam. This beam must be provided with a relatively large dimension in the direction transverse to material movement in order that all or a significant portion of the material be irradiated in a single pass; minimizing the number of passes, and decreasing the cost of the process. However, it is extremely difficult to accurately control the electron density, and hence the dose pattern distribution, in a columnar beam of large cross-section. Moreover, to effect a large dose in a single pass, the beam current must be relatively high, and a stationary columnar beam of high current presents severe window problems. Specifically, the continuous beam passing through a localized area in a window tends to heat the window to a point of weakened tensile strength whereby the window is subject to rupture by atmospheric pressure acting on one side against the interior vacuum on the other side.
The scanning beam approach was employed to minimize the effects of window heating produced by the stationary beam. Specifically, and as disclosed in U.S. Pat. No. 2,602,751, a narrow electron beam is caused to scan the treated material transversely of the direction of material motion. An elongated window is employed, and since the beam is continuously moving it does not severely heat a localized area of the window. The scanning approach requires precise electronic circuitry to effect the desired dose distribution pattern across the conveyed irradiated material. For example, a precise saw tooth scanning waveform must be generated if a uniform dose distribution is required. Non-uniform dose distributions require other precise, and sometimes irregular waveforms. Moreover, the beam scanning approach may be uneconomical even for uniform dose distribution, such as where the treated material requires a large dose for curing but can only tolerate a small dose rate. To achieve the required large dose in a single pass, the scanning beam must have a relatively high intensity and therefore may exceed the permissible dose rate of the material. Multiple passes at lower beam intensities are thus required to achieve the overall dose, and the cost of the process increases significantly.
The electron accelerator tube disclosed in U.S. Pat. No. 2,887,599 to Trump comprises an elongated cathode which emits a sheet or curtain of electrons across the transverse dimension of the conveyed material to be treated. This tube has the advantage of not requiring complex scanning circuitry. Moreover, since the electrons are issued as an extended sheet rather than as a columnar beam, the same total beam current may be achieved with a lower instantaneous density, thereby minimizing window heating. However, the Trump tube is limited regarding the dose distribution it can produce in the treated material; once the distribution pattern is set it cannot be changed. Moreover, Trump's tube, although better than the scanning beam tube, is also unable to supply a large dose at low dose rates in a single pass of the treated material.
It is therefore an object of the present invention to provide an electron accelerator tube devoid of the aforementioned disadvantages inherent in the prior art. More particularly it is an object of the present invention to provide an electron accelerator capable of providing a dose distribution pattern which is easily adjusted and also capable of delivering large doses at small dose rates in a minimum number of passes of treated material.
It is another object of the present invention to provide an electron accelerator which is particularly suitable for irradiating conveyed thin film material with an area beam having a dose distribution pattern which is controllable both transversely and longitudinally of the direction of material motion.